Circuit for generating high voltage pulse

ABSTRACT

A simple and less expensive high voltage pulse generating circuit including a low voltage direct current voltage source having one output terminal connected to another output terminal via a series circuit of a first switch with a low withstand voltage, an inductance storing inductive energy and a second switch with a high withstand voltage, and a branch circuit including a free-wheel diode connected between the other output terminal of the direct current voltage source and a common connection point between the first switch and the inductance. After storing inductive energy in the inductance by turning “on” the first and second switches, these first and second switches are turn “off” to commutate the energy stored in the inductance into a capacitive load connected across the second switch to charge the load abruptly and generate a high voltage pulse having a very narrow width without using a complicated and expensive magnetic compression circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit for generating a high voltagepulse having an extremely high voltage and a large content with the aidof semiconductor switches.

2. Related Art Statements

In order to generate plasma, an abruptly raising high voltage pulse ofseveral kV to several tens kV having a very short duration (sometimes 50nano seconds is required) has to be applied to a load, i.e. a dischargegap provided in a plasma generating reactor.

FIG. 1 is a circuit diagram showing a principal structure of a knownhigh voltage generating circuit. A DC supply source 1 having a highoutput voltage which is equal to a voltage of an output high voltagepulse is connected across a pulse energy supplying capacitor 3 via acharging resistor 2. The capacitor 3 is connected across a load(discharging site) 5 via a switch 4. After charging the capacitor 3,when the switch 5 is made conductive, energy is transferred from thecapacitor 3 to the load 5.

An inductance existing in a path of a current flowing from the capacitor3 to the load 5 through the conducting switch 4 is denoted by aninductance 6 in FIG. 1. The load 5 is formed by the discharge gap and itgenerally consisting of a capacitive element. In FIG. 1, for the sake ofexplanation, this capacitive element of the load 5 is denoted by acapacitor 7 connected in parallel with the discharge gap 5. When theswitch 4 is made conductive, a current flows to the capacitor 7 and thecapacitor is charged. The larger and steeper this current is, thesteeper the output pulse generated across the capacitor 7 becomes. Inthis manner, a preferable pulse for the plasma discharge can beattained. However, in practice, the switch 4 has a finite switching timeand could not be made conductive instantaneously, and the relativelylarge inductance 6 is always existent in the circuit. Therefore, araising edge of the output pulse could not be steep and an output pulsehaving a short duration could not be generated.

In order to solve the above explained problem of the known pulsegenerating circuit, there has been proposed a magnetic compressioncircuit utilizing a saturable iron core. FIG. 2 illustrates such amagnetic compression circuit. In FIG. 2, elements similar to those shownFIG. 1 are denoted by same reference numerals used in FIG. 1 and theirdetailed explanation is dispensed with. A series circuit of saturableiron or magnetic cores 8-1, 8-2 and 8-3 is connected across the switch 4and the load 5, capacitors 3-1, 3-2 and 3-3 are connected betweenterminals of these saturable iron cores and a negative terminal of theDC supply source 1, and a saturable iron core 8 is connected across theload 5.

An inductance of the saturable iron core is very high until the core issaturated, and when a product of a voltage and time reaches apredetermined value, the inductance of the saturable iron core decreasesabruptly. For the sake of explanation, it is assumed that inductancevalues of the saturable iron cores 8-1, 8-2, 8-3 and 8 are decreased inthis order and the capacitors 3-1, 3-2 and 3-3 have a same capacitancevalue. After the switch 4 has been made conductive and the saturableiron core 8-1 has been saturated at an instance T₀, voltage pulses v1,v2 and v3 appearing across the capacitors 3-2, 3-3 and 7 aresuccessively compressed on a time axis as depicted in FIG. 3. That is tosay, the voltage pulse v1 appearing across the capacitor 3-2 begins toincrease from the Instance T₀ and becomes maximum after a time durationT₁. Since the circuit Is designed such that the saturable iron core 8-2is saturated at such a time instance, the voltage pulse v2 appearingacross the capacitor 3-3 begins to raise and becomes maximum after atime period T₂. At this time, the saturable iron core 8-3 is saturatedand the voltage pulse v3 begins to raise. After a time period T₃ whichis shorter than the time period T₂, the voltage pulse v3 reaches amaximum value. In this manner, the voltage pulse v3 having a sharpraising edge as well as a relatively short pulse width can be appliedacross the load 5.

As illustrated in FIG. 2, the known high voltage pulse generatingcircuit including the saturable reactors has a complicated construction.Since a high voltage is applied to all the elements in the circuit, itis required to use special parts, and it is also required to provide alonger insulation distance. Moreover, the DC supply source 1 has togenerate a high voltage. In this manner, the known circuit is liable tobe large in size and expensive in cost.

In the known high voltage pulse generating circuit, the switch 4 isgenerally formed by a thyratron which is a kind of vacuum tube. Sincethe thyratron has a very high switching speed and can be used under ahigh voltage, the switch 4 can be formed by a single thyratron, andtherefore an inductance of the switch 4 is small. However, the thyratronhas the following demerits:

(1) The thyratron cannot operate at a high repetition frequency.

(2) The thyratron cannot be self-turned off and thus a limitation isimposed upon designing the circuit.

(3) The thyratron has a short lifetime and maintenance is cumbersome andexpensive.

(4) The thyratron requires a heater circuit as well as a gas control,and therefore the overall circuit is liable to be complicated.

(5) The thyratron malfunctions due to jitter and miss-ignition.

Recently semiconductor switches have been developed in accordance withthe progress of power electronics, and there have been designedsemiconductor switches which can turn-on and turn-off a large currentunder a high voltage. However, a semiconductor switch has a lowerwithstand voltage and could not be substituted for the thyratron. Aswitch is composed of a series circuit of a number of semiconductorswitches and a necessary circuit voltage is sheared by thesesemiconductor switches. In order to turn-on simultaneously thesemiconductor switches connected in series, it is necessary to providespecial gate driving circuits. Furthermore, a high voltage is appliedbetween the gate driving circuits, and therefore gate power sources andgate control signals have to be isolated from each other. In general, aremarkable advantage could not be attained by only replacing thethyratron by a series circuit of semiconductor switches.

Recently semiconductor switches have been developed in accordance withthe progress of power electronics, and there have been designedsemiconductor switches which can turn-on and turn-off a large currentunder a high voltage. However, a semiconductor switch has a lowerwithstand voltage and could not be substituted for the thyratron. Aswitch is composed of a series circuit of a number of semiconductorswitches and a necessary circuit voltage is shared by thesesemiconductor switches. In order to turn-on simultaneously thesemiconductor switches connected in series, it is necessary to providespecial gate driving circuits. Furthermore, a high voltage is appliedbetween the gate driving circuits, and therefore gate power sources andgate control signals have to be isolated from each other. In general, aremarkable advantage could not be attained by only replacing thethyratron by a series circuit of semiconductor switches.

As explained above, in the known high voltage pulse generating circuit,a high DC voltage source is required and all the circuit components aresubjected to a high voltage. Moreover, a pulse having a short widthcould not be produced due to a limitation in switching speed and acircuit inductance, and therefore the magnetic compression circuit hasto be used. Then, the circuit becomes large and expensive.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a simple and lowcost high voltage generating circuit which can generate directly anarrow high voltage pulse raising sharply without using the magneticcompression circuit by effectively utilizing the circuit inductance.

It is another object of the invention to provide a high voltagegenerating circuit which can generate a narrow and steep high voltage bymeans of semiconductor switches having turn-off faculty and operatingwith a relatively low DC voltage source.

According to the invention, a high voltage pulse generating circuitcomprises:

a DC voltage source having first and second output terminals;

a first switch having one end connected to said first output terminal ofsaid DC voltage source;

a branch circuit including a free-wheel diode connected across the otherend of said first switch and said second output terminal of the DCvoltage source; and

a series circuit including an inductance and a second switch andconnected in parallel with said branch circuit;

wherein after making said first and second switch on to store inductiveenergy in said inductance, the energy stored in the inductance iscommuted to a load connected across said second switch by turning-offsaid first and second switches.

In the high voltage generating circuit according to the invention, saidfirst and second switches may be formed by first and secondsemiconductor switches. In such a case, a low DC voltage is applied tothe inductance through the first and second semiconductor switches tostore inductive energy in the inductance, and then the first and secondsemiconductor switches are turned-off to commutate the inductive energyto a load capacitance of a low inductance circuit. By charging the loadcapacitance abruptly, it is possible to generate a high voltage pulsehaving a narrow width.

In a preferable embodiment of the high voltage pulse generating circuitaccording to the invention, said first semiconductor switch isconstituted by a semiconductor switching element having a low withstandvoltage and said second semiconductor switch is constructed by a seriescircuit of a plurality of semiconductor switching elements having a highwithstand voltage, the number of said plurality of semiconductorswitching elements being determined in accordance with an amplitude ofan output voltage pulse to be generated. There are further provided aplurality of iron cores, the number of which is equal to that of saidplurality of semiconductor switching elements. A primary winding passingthrough said plurality of iron cores is connected in series with saidfree-wheel diode, and a plurality of secondary windings each passingthrough respective iron cores are connected to gates and cathodeterminals of respective semiconductor switching elements of said seriescircuit of semiconductor switching elements. In this case, it isparticularly preferable that each of the semiconductor switchingelements of said series circuit is formed by a static inductionthyristor. However, according to the invention, the semiconductorswitching elements may be formed by another semiconductor switchingelement such as an insulated gate bipolar transistor (IGBT) which has aturn-off faculty.

In a preferable embodiment of the high voltage pulse generating circuitaccording to the invention, after discharging the energy to the load byturning-off the second switch, the second switch is turned-on again fora very short time period. Furthermore, said first and second switchesmay be turned off simultaneously or at different timings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a principal structure of a knownhigh voltage pulse generating circuit;

FIG. 2 is a circuit diagram illustrating a known high voltage pulsegenerating circuit including a magnetic compression circuit;

FIG. 3 is a signal waveform explaining the known high voltage pulsegenerating circuit shown in FIG. 2;

FIG. 4 is a circuit diagram depicting a first embodiment of the highvoltage pulse generating circuit according to the principal conceptionof the invention;

FIGS. 5A-5H are signal waveforms representing the operation of the highvoltage pulse generating circuit illustrated in FIG. 4;

FIG. 6 is a circuit diagram showing a second embodiment of the highvoltage pulse generating circuit according to the invention;

FIG. 7 is a circuit diagram illustrating a third embodiment of the highvoltage pulse generating circuit according to the invention;

FIG. 8 is circuit diagram depicting a fourth embodiment of the highvoltage pulse generating circuit according to the invention; and

FIG. 9 is a circuit diagram showing a fifth embodiment of the highvoltage pulse generating circuit according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 is a circuit diagram showing a first embodiment of the highvoltage pulse generating circuit according to the principal conceptionof the present invention. There is arranged a low DC voltage source 11whose output voltage can be determined regardless of an amplitude of anoutput high voltage pulse to be generated. A positive output terminal ofthe DC voltage source 11 connected to its negative output terminal bymeans of a series circuit of a first switch 12 having turn-on andturn-off faculty and a lower withstand voltage, an inductance 16 forstoring a inductive energy, and a second switch 14 having turn-on andturn-off faculty and a higher withstand voltage. The first switch 11having the turn-on and turn-off faculty serves to perform the supply andstop of the inductive energy to the inductance 16 and can be formed by aswitching element having a lower withstand voltage. The second switch 14also having the turn-on and turn-off faculty operates to perform thesupply and release of the inductive energy of the inductance 16 and theoutput high voltage is applied to the second witch. Therefore, thesecond switch 14 should have a higher withstand voltage than that of thefirst switch 12.

A branch circuit including a free-wheel diode 13 is connected across acommon connection point between said first switch 12 and said inductance16 and the negative output terminal of the DC voltage source 11. Sincethe high output voltage is not applied to the free-wheel diode 13, thisdiode may be of a lover withstand voltage. In parallel with the secondswitch 14, is connected a load 15, which may be a discharge gap providedin a plasma generating reactor. A capacitor 17 representing thecapacitive load 15 is shown in FIG. 4.

Now the operation of the high voltage pulse generating circuit accordingto the invention will be explained with reference to FIG. 5. FIG. 5Arepresents an on-off condition of the first switch 12, FIG. 5B an on-offcondition of the second switch 14, FIG. 5C a current i_(sw1) flowingthrough the first switch 12, FIG. 5D a circuit i_(L) passing through theinductance 16, FIG. 5E a current i_(sw2) flowing through the secondswitch 14, FIG. 5F a current i_(D) flowing through the free-wheel diode13, FIG. 5G a resonance current i_(c) passing through the capacitor 17,and FIG. 5H shows a voltage appearing across the capacitor 17, i.e. theoutput high voltage pulse Vc.

Now it is assumed that the first and second switches 12 and 14 areturned-on at a timing t₀. It should be noted that according to theinvention, the second switch 14 may be turned-on prior to the firstswitch 12. Then, a low voltage E of the DC voltage source 11 is appliedto the inductive energy storing inductance 16 (having an inductancevalue L) and the current i_(L) passing through the inductance 16increases linearly with an inclination of E/L (FIG. 5C). That is to say,the inductive energy is stored in the inductance 16. In a first mode Ishown in FIG. 5H, this current i_(L) is equal to the current i_(sw1)passing through the first switch 12 and the current i_(sw2) passingthrough the second switch 14.

When the current passing through the inductive energy storing inductance16 reaches a given current Ip at an instance t₁, the first and secondswitches 12 and 14 are turned-off (FIGS. 5A and 5B). In this case, thefirst and second switches may be turned-off at different timings, butfor the sake of explanation, the first and second switches areturned-off simultaneously. In a second mode 11 which is initiated at theturn-off of the first and second switches 12 and 14, the current flowingthrough the inductive energy storing inductance 16 is commutated to thebranch circuit including the free-wheel diode 13 as well as to the load15 (capacitor 17). That is to say, the inductive energy (LI_(p) ²/2)stored in the inductance 16 initiates a resonance along a loop ofinductance 16→capacitor 17→free-wheel diode 13.

The resonance current I may be represented by the following equation(1):

i−I_(p) cosωt  (1)

The voltage Vc across the capacitor 17 is denoted by the followingequation (2): $\begin{matrix}{{{V\quad c} = {{\frac{J_{P}}{\omega \quad C}\sin \quad \omega \quad t} = {V_{P}\sin \quad \omega \quad t}}}{{Here},}} & (2) \\{{\omega - \frac{1}{\sqrt{L\quad C}}} = \frac{\pi}{2T_{1}}} & (3)\end{matrix}$

In general, a waveform of the output voltage pulse is determined inaccordance with the load 15. That is to say, a peak value V_(p) (whichcorresponds to a discharge start voltage) of the output voltage Vcapplied to the load 15 and a time duration T₁ from the time instance t₁to a time instance at which the output voltage reaches its peak (a timeinterval of the second mode ID are given, and furthermore thecapacitance C of the capacitor 17 is determined by the load 15.Moreover, the output voltage E of the DC voltage source 11 can bedetermined at will and may be set to a suitable value for the systemunder consideration.

Therefore, from the above mentioned equations (1)-(3), L and Ip may beobtained by the following equations; (4) and (5), respectively:$\begin{matrix}{L = {\frac{1}{C}\lbrack \frac{2T_{1}}{\pi} \rbrack}^{2}} & (4) \\{{Ip} = \frac{\pi \quad C\quad V_{p}}{2T_{1}}} & (5)\end{matrix}$

in order to flow the current I_(p) to the inductive energy stone,inductance 16, a time period T₀ during which both the first and secondswitches 12 and 14 are made conductive is set in the following manner:$\begin{matrix}{T_{0} = {\frac{L\quad I_{p}}{E} = \frac{2V_{p}T_{1}}{\pi \quad E}}} & (6)\end{matrix}$

Then, it is possible to obtain the high voltage pulse having a verynarrow width. By controlling the time duration T₀ during which both thefirst and second switches are made conductive, a peak value of theoutput pulse can be adjusted freely without changing a width T₁ of araising portion.

The output voltage Vc reaches its peak value at a time instance t₂, anda most efficient operation can be attained when the discharge isinitiated in the load 15 at this timing t₂. However, the discharge is avery complicated phenomenon which depends upon temperature, humidity andgas condition, and it is quite difficult to explain the dischargingoperation quantitatively. Therefore, the explanation of the discharge isdispensed with in the present specification. In general, if a decreasein the output voltage Vc due to the discharge is not abrupt, undesiredinfluence is applied to the discharge in a physical meaning. Therefore,it is advantageous to provide a mode IV in which the second switch 14 isturned-on at a time instance t₃ to decrease the output voltage Vc tozero forcedly.

As explained above, in the high voltage pulse generating circuitaccording to the invention, an extremely high voltage pulse can begenerated by the very simple circuit using a less expensive and smalllow voltage DC source by effectively utilizing the semiconductorswitches having the nun-off faculty instead of the thyratron which doesnot have a turn-off function. Furthermore, it is an important merit ofthe circuit according to the invention that the inductance of a circuitportion including the second switch 14 do not affect principally thegeneration of the output voltage pulse.

FIG. 6 is a circuit diagram showing a detailed arrangement of a secondembodiment of the high voltage pulse generating circuit according to theinvention. In FIG. 6, portions having similar functions as those of FIG.4 are denoted by same reference numerals used in FIG. 4. To a DC voltagesource 21 is connected a direct current smoothing circuit consisting ofan inductor 22 and a capacitor 23 such that a high frequency impedanceof the voltage source is sufficiently decreased and a pulse current canbe supplied smoothly. In the present embodiment, the first switch 12shown in FIG. 4 is formed a power MOSFET 24. As explained before, thefirst switch may have a lower withstand voltage, but it may beconstructed by a parallel circuit of a plurality of power MOSFETs inaccordance with a peak value Ip of the current passing through them. Thesecond switch 14 shown in FIG. 4 is formed by a series circuit of aplurality of static induction thyristors 25-1˜25-4. The seriesarrangement of these static induction thyristors 25-1˜25-4 is denoted by25. The number of the static induction thyristors 25-1˜25-4 in theseries arrangement is determined by the peak value V_(p) of the outputvoltage pulse as well as by withstand voltages of these static inductionthyristors. In the present embodiment, four static induction thyristors25-1˜25-4 are provided.

As stated above, when a plurality of the controllable semiconductorswitching elements, in the present embodiment the four static inductionthyristors 25-1˜25-4 are connected in series, these semiconductorswitching elements require respective gate driving circuitsindependently. Since the high voltage (maximum voltage is V_(p)) isapplied between respective gaze driving circuits, a high withstandvoltage isolation has to be provided between voltage source and controlsignals for these gate driving circuit. This results in an increase insize and cost of the circuit as well as in a decrease in reliability. Inorder to share the high voltage by the semiconductor switching elementsequally, it is necessary to turn-on and turn-off these semiconductorswitching elements abruptly. To this end, gate signals should be appliedto the semiconductor switching elements simultaneously in a very precisemanner. This requires a highly developed technique.

In the present embodiment, in order to solve the above mentionedproblem, the static induction thyristors are not provided withrespective gate driving circuits. That is to say, in the presentembodiment, there are arranged a plurality of iron cores 26-1˜26-4, thenumber of which is identical with that of the static inductionthyristors 24-1˜24-4, and a primary winding 27 constructed by the branchcircuit including the free-wheel diode 13 is passed through the ironcores 26-1˜26-4. Secondary windings 28-1˜28-4 each connected acrossgates and cathodes of respective static induction thyristors 24-1˜24-4are passed through respective iron cores 26-1˜26-4. In this manner, theprimary winding 27 of a single turn are provided commonly for all themagnetic cores 24-1˜24-4 and the secondary windings 28-1˜28-4 of asingle turn are provided for respective magnetic cores 24-1˜24-4.

Next the operation of the second embodiment of the high voltage pulsegenerating circuit according to the invention will be explained withreference to FIG. 5. At a time instance t₀, the power MOSFET 24 isturned-on and a current flows from the capacitor 23 of the directcurrent smoothing circuit to the parallel circuit of a capacitor 29 andresistor 30 via the power MOSFET 24 and magnetic cores 261˜264. Thecapacitor 29 operates as a speed-up capacitor for flowing a largecurrent immediately after turning-off of the power MOSFET 24. Theresistor 30 serves to flow an intermittent current. The same currentflows in the secondary windings 28-1˜28-4 coupled with the magneticcores 26-1˜26-4 in such a direction that a magnetic flux induced by thecurrent flowing through the primary winding 27 is cancelled out Thesecurrents serve as on gate currents for the static induction thyristors25-1˜25-4, and these static induction thyristors are made onsimultaneously 4 In this manner, the power MOSFET 24 and staticinduction thyristors 25 (all the static induction thyristors 25-1˜25-4)are made conductive, and the current flows to the inductive energystoring inductance 16. After that, the circuit operates in the samemanner as that of the above explained first embodiment Here, the currentflowing to the inductive energy storing inductance 16 does not raiseabruptly, and it is not necessary to turn-on the static inductionthyristors 25-1˜25-4 abruptly. Therefore, it is not always necessary toprovide the capacitor 29, in which case only the resistor 30 may bearranged.

However, the turn-off operation of the static induction thyristors 25constituting the second semiconductor switch differs from the firstembodiment as will be explained hereinafter. When the power MOSFET 24constituting the first semiconductor switch is turned-off at a timing t₁at which the current passing through the inductive energy storinginductance 16 becomes the maximum value I_(p), the current which hasflown to the inductive energy storing inductance 16 is commutated to thebranch circuit including the free-wheel diode 13. This current flowingalong the primary winding 27 coupled with the magnetic cores 26-1˜26-4,and currents having the same amplitude as that flowing through theprimary winding flow through the secondary windings 28-1˜28-4 in such adirection that the magnetic flux induced by the current flowing throughthe primary winding is cancelled out. These currents constitute gateturn-off currents for the static induction thyristors 25-1˜25-4, andthese static induction thyristors are turned-off simultaneously. Itshould be noted that the static induction thyristors 25-1˜25-4 servingas the second switch have a relatively high withstand voltage and canturn-on and -off at a high speed. Such a static induction thyristor inthe current driven device instead of the voltage driven device such asIGBT, and the larger the turn-on gate current and turn off gate currentis, the faster the turn-on and turn-off operation is performed.Therefore, such a static induction thyristor is preferably utilized inan application requiring a fast operation such as a pulsed powerapplication. When a turn-off gain (a ratio of the anode current to beturned-off to the gate turn-off current) is small, a storage time duringthe turn-off of the semiconductor switch can be shortened and a falltime can be also shortened. Therefore, the static induction thyristorcan be particularly preferably used in the case in which anode currentis identical with the gate turn-off current and the turn-off gainbecomes unity like as the second embodiment.

In the second embodiment mentioned above, the gate turn-on current andgate turn-off of the static induction thyristors 25-1˜25-4 are identicalwith each other and are large, and thus the static induction thyristorscan be simultaneously turned-on and turned-off without time differenceat a high speed. In this manner, the turn-on and -off operation can beperformed reliably without any gate driving power source as well asindependent gate signals.

In the first and second embodiments, at a suitable timing t₃ after theload 15 has initiated the discharging, the second semiconductor switch14; 25 may be turned-on again to discharge the capacitor 17 at a highspeed. This becomes an effective means for such a case that a dischargeimpedance of the load 15 is too high to discharge the capacitor 17 at ahigh speed and the physical phenomenon of the discharge is affected. Thesecond semiconductor switch 14; 25 may be turned-on again by turning onthe second semiconductor switch for a very short time period asillustrated by a broken line in FIG. 5B. Then, the output pulse voltageVc can be decreased to zero instantaneously as shown by a broken line inFIG. 5H.

In the second embodiment, the first semiconductor switch is formed bythe power MOSFET 24 and the second semiconductor switch is constructedby the static induction thyristors 25-1˜25-4. It should be noted thatthese semiconductor switches may be formed by any other semiconductorswitching element such as another type of transistors and IGBT (when itis used as the second semiconductor switch, a care should be taken in apoint that it is a voltage driven device and a limitation is imposedupon a gate-emitter voltage). Furthermore, in the second embodiment, thesecond semiconductor switch is constituted by the series circuit of thefour static induction thyristors 25-1˜25-4, but according to theinvention, the number of static induction thyristors is determined by apeak value of the output pulse voltage. It is a matter of course thatthe load is not limited to the capacitive discharge circuit. Moreover,the first switch 12 and inductive energy storing inductance 16 areconnected to the positive output terminal of the direct current voltagesource, but according to the invention, the same function can beattained by connecting the first switch and inductive energy storinginductance to the negative output terminal as shown in FIG. 7.Alternatively, one of the first switch 12 and inductive energy storinginductance 16 may be connected to the negative output terminal of thevoltage source.

FIG. 8 shows a fourth embodiment of the high voltage pulse generatingcircuit according to the invention, in which a first switch 12 isconnected to a positive output terminal of the DC voltage source 11 anda inductive energy storing inductance 16 is connected to a negativeoutput terminal of the DC voltage source 11.

FIG. 9 depicts a fifth embodiment of the high voltage pulse generatingcircuit according to the invention, in which a first switch 12 isconnected to a negative output terminal of the DC voltage source 11 anda inductive energy storing inductance 16 is connected to a positiveoutput terminal of the DC voltage source 11.

In the above explained first to fifth embodiments of the presentinvention, each of the first and second switches are formed by asemiconductor switch and it is possible to generate a high voltageoutput pulse having an amplitude of several kVs to several tens kV and apulse duration of several tens nano seconds to several hundreds of nanoseconds.

What is claimed is:
 1. A high voltage pulse generating circuitcomprising: a DC voltage source having first and second outputterminals; a first semiconductor switch having a low withstand voltagewith turn-on and turn-off faculty and having one end connected to saidfirst output terminal of said DC voltage source; a branch circuitincluding a free-wheel diode connected across the other end of saidfirst semiconductor switch and said second output terminal of the DCvoltage source; and a series circuit connected in parallel with saidbranch circuit and including an inductance and a second semiconductorswitch with turn-on and turn-off faculty, said second semiconductorswitch being constructed by a series circuit of a plurality ofsemiconductor switching elements having a high withstand voltage, thenumber of which is determined in accordance with an amplitude of anoutput voltage pulse to be generated, said circuit further comprises aplurality of iron cores, the number of which is equal to that of saidplurality of semiconductor switching elements, a primary winding passingthrough said plurality of iron cores and being connected in series withsaid free-wheel diode and a plurality of secondary windings each passingthrough respective iron cores and being connected to gates and cathodeterminals of respective semiconductor switching elements of said seriescircuit of semiconductor switching elements; wherein after turning saidfirst and second switches on to store inductive energy in saidinductance, the energy stored in the inductance is commutated to a loadconnected across said second semiconductor switch by turning said firstand second semiconductor switches off.
 2. The high voltage pulsegenerating circuit according to claim 1, wherein each semiconductorswitching element of said series circuit of a plurality of semiconductorswitching elements constituting said second semiconductor switch isformed by a static induction thyristor.
 3. The high voltage pulsegenerating circuit according to claim 2, wherein said primary windingand secondary windings are wound on the iron cores by one turn.
 4. Thehigh voltage pulse generating circuit according to claim 2, wherein saidfirst semiconductor switch having a low withstand voltage is formed by apower MOSFET.
 5. The high voltage pulse generating circuit according toclaim 1, wherein said first and second semiconductor switches are turnedoff substantially simultaneously.
 6. The high voltage pulse generatingcircuit according to claim 1, wherein said second semiconductor switchis turned off immediately after said first semiconductor switch isturned off.
 7. The high voltage pulse generating circuit according toclaim 1, wherein a parallel circuit of a capacitor and a resistor isconnected in parallel with said free-wheel diode.
 8. The high voltagepulse generating circuit according to claim 1, wherein a resistor isconnected in parallel with said free-wheel diode.
 9. The high voltagepulse generating circuit according to claim 1, wherein said load is adischarge gap provided in a plasma generating reactor.
 10. A highvoltage pulse generating circuit comprising: a DC voltage source havingfirst and second output terminals; a first switch having one endconnected to said first output terminal of said DC voltage source; abranch circuit including a free-wheel diode connected across the otherend of said first switch and said second output terminal of the DCvoltage source; and a series circuit including an inductance and asecond switch and being connected in parallel with said branch circuit;wherein after turning said first and second switches on to storeinductive energy in said inductance, the energy stored in the inductanceis commutated to a load connected across said second switch by turningsaid first and second switches off; and wherein said second switch isturned on again after turning off the second switch to discharge energyremaining in said circuit.
 11. The high voltage pulse generating circuitaccording to claim 10, wherein said second switch is turned on again fora short time period after turning off the second switch to dischargeenergy remaining in said circuit.